Geochemistry, Geophysics, Geosystems

Transfer of atmospheric boron from the oceans to the continents: An investigation using precipitation waters and epiphytic lichens



[1] The variation of B concentration in atmospheric deposition was studied from the analysis of 35 individual rain events, 17 snow packs, and 17 lichens sampled over NE North America (south from Hudson Bay) and Asia (from the coast of Bangladesh to the high Himalayas of Nepal). Rain samples show a range of B concentration between 0.3 and 9.4 μg/L (average of 1.8 ± 1.7 μg/L), excluding two rains with higher B contents of 17 and 37.5 μg/L, most likely reflecting anthropic contamination. Snowpacks and lichens which average atmospheric deposition over periods of a few months to a few years show a smaller range of variation, from 0.1 to 2.3 μg/L (average 1.1 ± 0.8 μg/L) for snowpacks and from 1 to 25.9 ppm for lichens. The lichens have elemental ratios (such as B/Cl) similar to the average of rains, showing that they are good monitors of atmospheric B deposition without significant biofractionation of elements. This is also demonstrated by their halogen contents, which follow the systematics of the atmospheric distribution of these elements previously derived from the study of rains and atmospheric particles. Though individual rains do not show systematic decrease in B concentration with distance to the sea, this behavior is clearly shown by samples having longer integration times, snowpacks, and lichens. The snow and lichen data show that seawater is a major source of atmospheric B. Using the lichen data, the enrichment factor (normalized to Na) of marine air masses relative to seawater is estimated to be ∼13 (average B/Na of 5.6 × 10−3). This implies that the fractionation factor of atmospheric suspended marine particles relative to seawater (FX = (X/Na)particle/(X/Na)seawater) is 0.177. Thus B transfer from seawater to the atmosphere occurs mostly via degassing of sea salts, the residence time of gaseous B being estimated at 15.9-fold that of particulate B. The distribution of B in the atmosphere between gaseous B and degassed sea salts can be used to model the large range of B concentrations measured in single rain events by selective removal of one of the two B components. In this model, unfractionated marine air masses can reach the continents with very variable B concentrations, between ∼0.2 and ∼20 μg/L.